You could explicitly set a style for your app using QApplication::setStyle() which would make your app look roughly the same on each platform, but would probably make it look "different" from a native app. I believe that the default drawing of primitives (buttons, backgrounds, highlights, etc.) is done using the platform's native API, so any differences between XP and Win 7 are the result of native implementations on each.

You could also play around with the palette for the app (or for individual widgets if an app-wide change in undesirable), changing the QPalette::Window color, for example.